1. Field of the Invention
The present invention relates generally to digital communication techniques. More specifically, the invention relates to a system and method for balancing the amplitude and phase of a received, quadrature-phase modulated signal.
2. Description of the Prior Art
One of the common methods for modulating digital signals is the use of multilevel systems or M-ary techniques. M-ary modulation techniques are natural extensions of binary modulation techniques and apply to L-level amplitude or phase shift keying. A commonly used quadriphase scheme is called quadrature phase shift keying or QPSK. Like all of the M-ary amplitude or phase schemes, its principle advantage is bandwidth reduction.
Since pulse rate fp is:fp=fs logLM,  Equation 1where fs is the symbol rate and M is the number of messages; with L representing the number of modulation levels, the larger L is, the smaller the pulse rate and hence, the smaller the bandwidth.
In telecommunication applications, QPSK modulates two different signals into the same bandwidth creating a two-dimensional signal space. This is accomplished by creating a composite phase modulated signal using two carriers of the same frequency but having a phase difference of 90 degrees as shown in FIG. 1A. By convention, the cosine carrier is called the in-phase component I and the sine carrier is the quadrature component Q. The I component is the real component of the signal and the Q component is the imaginary component of the signal. Each of the I and Q components are bi-phase modulated. A QPSK symbol consists of at least one sample from both the in-phase I and quadrature Q signals. The symbols may represent a quantized version of an analog sample or digital data.
All phase modulated schemes must overcome the inevitable problem of phase synchronization. For proper operation of QPSK signaling, the I and Q channels should have the same gain throughout processing both received channels, keeping the I and Q channels uncorrelated. Mismatched signal gains or magnitudes between the uncorrelated I and Q channels create errors when processing. Phase differences other than 90 degrees between the signals cause spillover between the channels and similarly result in degraded performance.
Typical receivers exhibit different overall gains for the separate I and Q channels due to mismatched gains in the mixers, filters, and A/D converters caused by variations in component values due in part to temperature, manufacturing tolerances and other factors. Amplitude and phase imbalance between the I and Q channels result in the distortions shown in FIGS. 1B and 1C, decreasing overall signal-to-noise ratio (SNR).
Prior art approaches taken to avoid amplitude and phase imbalance rely upon very precise circuitry controlling each gain stage with active temperature compensation. These expensive designs require components that are manufactured with extremely low temperature coefficients and with the mixers for the I and Q channels custom matched during manufacture.
Accordingly, there exists a need for a system that balances the amplitude and phase of a QPSK signal upon reception increasing signal integrity and thereby reducing bit error rate (BER).